Method for producing conducting coils



March 27, 1956 R. o. GRISDALE ET A1. 2,739,371

METHOD FOR PRODUCING CONDUCTING COILS Filed Aug. 4. 1951 2 Sheets-Sheetl /A/ VEN T ORS 6;,0 GR/SAL E A.SAUER A 7` TOR/VE V March 27, 1956 R. o.GRISDALE ET Al.

METHOD FOR PRODUCING CONDUCTING COILS 2 Sheets-Sheet 2 Filed Aug. 4.1951 FIG. 3

ATTORNEY METHD FR PRDUCNG CONDUCTING COILS Richard Grisdale, ShortHilis, N. J., and Harold A. Sauer, Warminster, Pa., assiwors to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Appiication August 4, V1951, Serial No. 240,412

3 Claims. (Cl. 29-15557) This invention relates to coils and methods oftheir manufacture and more particularly to coils having their turnsembedded in ya matrix, otherwise known as filled coils, and theirmethods of manufacture.

Heretofore in the manufacture of high quality electrical coils it hasbeen commony practice to insulate and provide for the mechanicalseparation of the individual turns, and then to impregnate the entirecoil structure with a filler in operations which 'are 'separate anddistinct from that of winding the coil. Thus, the conductor employed forthe winding usually is coated with a relatively high quality coatingwhich is sufliciently tough and abrasion resistant so that it can berespooled and otherwise handled prior to the winding operation withoutdamage. ln effecting the mechanical separation of adjacent turns andlayers of coill turns made up of the coated conductor, the coating andsheets of insulating material between adjacent layers are usually reliedupon. Following the winding operation, the coil is impregnated withfiller material which is injected in its interstices and mechanicallyset to improve the insulation between turns and to bond the coil into aunitary structure.

It has also been suggested that the separate impregnating step can beeliminated in the manufacture of high quality coils by employing avconductor which has been covered with a porous material such asasbestos, paper, cotton or other fibrous material, or with a layer whichis impermeable to liquids such as insulating enamel and by applying aplastic coating to the covered conductor immediately preceding thewinding of the coil. The plastic coating of this construction, due toits initial fluidity, replaces the ller impregnant of the coilsdescribed above in that it fills the coil interstices, improves theelectrical insulation between turns and when solidified bonds adjacentturns and layers of turns together, while the initial conductor coveringprovides the means for insuring mechanical spacing between turns'.

Objects of this invention are to improve and simplify the manufacturingprocess for and the construction of coils having their turns embedded ina substantially solid matrix.

A more specific object is to eliminate in the manufacture of coils theseparate operations of insulating, mechanically spacing, and bondingtogether of the conductor turns and of filling the coil interstices.Another object is to reduce the number of handling, prefabricating andstoring steps in manufacturing coils'.

Other objects of this invention are to reduce the number of elements incoil structures', to reduce the size of coil structures without anysacrifice in ytheir rated capacities or their operating qualities, toreduce the cost of coils, to increase their thermal conductivities, andto improve their copper eiciencies.

in accordance with features of this invention, a series of manufacturingsteps are so combined that they produce filled coils in a singlecontinuous series of operations. The initial operation in this processis the coating of 'the nited States Patent 2,739,371 Patented Mar. 27,1956 bare conductor which is to be utilized as the turns of the coil.The conductor at the instant it is incorporated into the coil by thewinding operation bears a coating which is of such a nature that itmechanically separates the adjacent turns of conductor, yet issufficiently fluid to till the interstices between adjacent coil turnsand to coalesce with the coatings on adjacent turns into a homogeneousmatrix. This condition of the coating is achieved by a combination offactors including its composition, its method of application and itstreatment in the interval betweenl its application and the winding ofthe conductor into the coil.

A feature of this invention resides in producing coils embedded in asubstantially solid matrix, in a continuous operation beginning with abare conductor.

Another feature resides in utilizing resins applied as coatings to theconductor immediately preceding its being wound into a coil as the meansfor maintaining the mechanical separation, the electrical insulation andthe bonding together of adjacent coil turns and as a coil filler wherebya more compact and continuous mass of embedded coil turns is produced.`

In accordance with one feature of this invention, the mechanicalseparation of the coil turns by the coating is insured by impartingsufficient mechanical set to at least a portion thereof so that it willmaintain itself between the coil turns.

This condition can be advantageously achieved by applying a plurality ofcoating layers to the bare conductor, the initial coating layer orlayers being solidified or set mechanically as by heating or othercuring techniques prior to the application of subsequent layers. Whenthe process is practiced in this manner, the outer portion of the'conductor coating provides sufficient iiuid material to ll theinterstices between the 'turns and to bond adjacent turns together 'asthe winding progresses. After the conductor is incorporated in the coilstructure all of the coating material is solid'ied.

In accordance with another feature of this invention, the conductorturns of a c'oil are maintained mechanically separate by finely dividedsolid insulating particles positioned between them. These separatingparticles may be incorporated into the structure by applying them to theconductor in a fluid Vehicle which also serves as the lilli-ng andbonding material of the finished coil. This combination permits themechanical setting of the uid, at least t'o any extent in excess of thatnecessary to maintain the spacing particles on the conductor, to bedelayed until the ycoil has been wound, thereby insuring the free ow offiller into the interstices of the coil and the establishment of ahomogeneous matrix comprising a dispersion of the spacing particles inthe solidified vehicle.

Another feature of this invention resides in curing or otherwiseeffecting a solidification or a mechanical set of the tller material inthe coil structure as it is built up so that the turns of the 'coil areembedded in a solid mass which requires no further treatment when theyare removed from the winding machine.

A further feature of this invention resides in a coil structure composedof adjacent turns embedded in a substantially homogeneous matrix. Thismatrix which is continuous and uninterrupted through the structureprovides a more rapid Conduction of heat to the exterior than ischaracteristic of those yprior structures containing multiple insulatinglayers on the conductors, voids between the conductors and interleavingbetween layers of conductor turns.

The invention, together with the above-noted and other objects andfeatures thereof, will be more fully understood from the followingdetailed description when read with reference to the accompanyingdrawings in which:

Fig. l is a block diagram of the method of this invention;

Fig. 2 is a representation of an apparatus used in carrying out themethod of this invention;

Fig. 3 is a front view, partially in section, of the coating cell shownin Fig. 2;

Fig. 4 is a perspective view of a roller applicator which may be used inlieu of the coating cell shown in Fig. 3; and

Fig. 5 is a partially sectioned View of a coil constructed by the methodof this invention.

As is illustrated in the block diagram of Fig. l, in manufacturing awinding or coil according to this process, a continuous series ofoperations is practiced starting with an uninsulated conductor andterminating with a completed rigid self-supporting winding. Theseoperations broadly comprise applying one or more resinous coatings to abare conductor to produce thereon a surface which insures that the turnsof the conductor, when it is wound into a coil, will be electricallyinsulated and mechanically separated from each other and which alsoilled the interstices between the turns and bonds them together to forma unitary structure, winding the conductor into the desired coil form,coalescing the coatings of adjacent turns, and solidifying the coatings.

In producing a coil structure of the filled type from bare conductorfour basic problems present themselves; iirst, that of maintainingmechanical separation of the adjacent turns of bare conductor in thecoil structure; second, that of establishing a continuous electricallyinsulating coating between adjacent conductors; third, that of fillingthe interstices of the coil with insulating material, and; fourth, thatof bonding the spaced and insulated turns into a unitary structure.Mechanical separation of adjacent turns can be maintained even wheresurface tension elects of the uid liller material and the pressuresapplied by the winding operation tend to pull or otherwise force theturns together if a form of solid spacer is placed between all turns asthe structure is built up. In embodiments of the present invention, twoforms of mechanical spacer have been provided, each of which isuniformly distributed over the conductor surface to insure itsseparation from all adjacent turns, and each of which is applicable tobare conductor at a rate equal to the Winding speed of the coil in whichit is to be employed.

One form of mechanical spacer functioning as outlined above comprises alayer of insulating material applied by one of the processes and of oneof the types discussed hereinafter. Several layers of coating materialare employed when this type of spacing means is utilized, the initiallayer or layers being solidified sufficiently so that the mechanicalrigidity thereof is suicient to maintain a coherent coating on theconductor throughout the subsequent coil forming operations. Solidifyingto this extent can be accelerated in a convenient manner such as theapplication of heat or a solidifying agent. Immediately following thesolidifying of the initial coating, more material of the same or adifferent kind is applied and maintained in fluid form until theconductor is wound to provide for the filling and bonding of the coil.

Alternatively, tinely divided solid insulating material located betweenthe coil turns can be employed as mechanical spacing means. Particles ofthis material insure a separation between turns at least as great astheir diameter and thereby permit a layer of uid at least equally thickto remain between the adjacent turns in the wound structure. Solidspacing particles can be applied and maintained on the conductor byseveral means. For example, the particles can be applied from asuspension and maintained by an adhesive applied to the bare conductor,the particles can be attracted to and maintained on the conductor byelectrostatic means, or a suspension of the particles in a uid vehiclecan be applied. The first two methods of application require theaddition of a uid bonding and iilling material which also forms aninsulating ilm on the conductors between the spacing particles. Thisfluid coating can be applied either before or after the spacingparticles and advantageously is of a nature that it further aids inmaintaining the particles on the conductor. The latter method ofapplication is advantageous in that it eliminates the step of separatelyapplying the tiuid. When the spacing of the turns is accomplished bylinely divided particles the coating material can be applied in a singlecoating step, it can be incorporated in the coil in a more uid formthereby insuring more complete lling of the interstices, and the spacefactor of the coil can be increased through increase in copper crosssection relative to coil cross section. Since no continuous solidiiiediilm exists on the conductor turns, no extra layer providing for thebonding of adjacent turns is necessary, the fluid between the solidparticles performing the bonding and filling functions in addition toproviding a continuous insulating film over each conductor turn.

In preparing a bare conductor for use in certain embodiments of thisprocess, a minimum of cleaning is necessary, since after the process isinitiated there is no further handling of the conductor other than bythe apparatus for practicing the process and therefore the mechanicalproperties of the coating and the bond between it and the conductor arenot subject to the stresses that ordinary insulated conductorencounters. It has been found practicable, when the coating is appliedelectrophoretically, to employ conductor as it is supplied from themanufacturer and to embody an electrolytic cleaning as one step in thecontinuous sequence of operations.

Both thermosetting and thermoplastic materials generically referred toas resinous materials can be ernployed as the basic coating and areapplicable either to the bare conductor or to conductor which, in aprevious step of continuous process, has been insulated with a curedlayer of resinous material which may or may not be similar to that ofthe bonding layer. Both the thermosetting and the thermoplasticmaterials are suitable vehicles for suspended solid spacing particles.

Thermosetting material is usually in liquid form when applied althoughit can also be applied as a tinely divided i solid which is coalesced bymoderate heating; this process involves the cross-linking ofpolymerization of the material. Suitable thermosetting materials includefast curing phenolics, and epoxy resins when cured in the presence ofoxygen.

Thermoplastic materials can be applied as heated liquids or as finelydivided solids which are coalesced by heat. These materials should havesoftening temperatures above the operating temperatures of the coils inwhich they are employed. Polyamides, polyvinyl chloride, polyethylene,butadiene-styrene-acrylonitrile copolymers, butadiene-styrenecopolymers, acrylic resins, and fluorocarbon resins such aspolytetrauoroethylene and polymonochlorotrifluoroethylene all haveacceptable thermoplastic characteristics.

Solid insulating materials which can be employed conveniently in finelydivided form as spacer bodies between adjacent conductor turns mayinclude among other substances iinely divided silica, aluminum silicate,magnesium silicate, mica, lead chromate, lead titanate, titaniumdioxide, zinc oxide, iron oxide, aluminum oxide, copper oxide, talc, andcellulose nylon or other tilamentary iioc. Any of these materials eithersingly or in combination may be suspended in a fluid vehicle providingthe coil filling and bonding, and such vehicles hereafter will bereferred to as loaded liuids.

Any suitable coating method can be employed for applying the coatingmaterials in either the loaded or unloaded state to the conductor. Thus,when in a finely divided solid state, they can be applied from a liquidor gaseous suspension by electrophoretic or other suitable means. Whenthe suspending uid is water it can be removed from the deposited coatingby moderate heating of the conductor prior to the winding operation.Suspending iiuids such as diyinyl benzene and acrylonitrile need not beremoved when employed for example, with polyester and butadiene-styreneco-polymers, respectively, since they cross-link or polymerize with thesuspended material to become a part of the coating. Materials which arein the liquid state can also be electrophoretically applied bysuspending them as globules in a suitable immiscible liquid. However,the more common practice with liquid coating materials is to apply themby a drag, dip, or spray technique, for example, by a die-wipe processor by a roller coat process.

After a coating has been applied to the wire it may be solidified. Wherea plurality of layers is employed each layer may be solidified orpartially solidified prior to the application of further layers.Thermosetting coating materials are conveniently solidified by theapplication of heat while thermoplastic materials are first coalesced byheat and are then cooled below their melting temperatures. Either typeof coating must be conditioned so that its mechanical stability issutiicient to maintain it and any solid spacing particles suspendedtherein on the wire surface when the conductor is subjected to the nextstep in the coil manufacturing process.

When the bonding layer has been applied to the conductor, the conductoris wound into a coil with the coatings of adjacent turns in Contact inthe usual manner. A bond is formed between the coatings on adjacentturns of the winding either by virtue of the tackiness or fluidity ofthe partially set coatings or by applying heat to soften the coatingsthereby coalescing them. The coalesced coatings are then completely setby freezing the thermoplastic material or by heating the thermosettingmaterial to cure it. The coil is then in condition to be assembled withthe core, terminals, mounting means and other elements with which it isemployed. An armoring coat of plastic material can be applied over theouter layer of turns before the nal curing or as the final as semblystep if desired.

An example of the product resulting from the abovedescribed steps isshown in Fig. 5 from which it can be seen that the overlying layers ofturns of conductor 26 are embedded in a substantially continuous matrixof homogeneous material which maintains the turns mechanically separateand electrically insulated from each other in a rigid body 27.Homogeneous as applied here to the material of the matrix describes amass of material which is essentially uniform in composition throughoutalthough it may be composed of a resinous mixture or such a mixture withsolid insulating lparticles uniformly distributed therein. It is to benoted that no insulating interleaving is employed between the layers ofturns. The absence of interleaving improves the thermal conductivity ofthe structure since it is more compact, the heat paths being shorter andmore nearly continuous than heretofore. This improved thermalcharacteristic provides a higher rate of heat dissipation, thus makingthe coil structure suitable for operation at higher loads than for theusual coils of similar size. Another advantage of this constructionresides in the increased ratio of the cross-section area of bareconductor to the total crosssection area of the coil to improve thecopper efficiency. Further, voids are unlikely in this structure sincethe impregnating material is applied to each portion in the properamounts as it is built Vup and need not be forced into the completedstructure, thus resulting in an increased electrical breakdown strength.A solid construction as achieved here also creates a more rugged bodywhich can withstand high compressive forces.

Referring now to Fig. 2, typical apparatus is shown for constructing acoil from bare wire in a continuous operation according to thisinvention. A wire 1 which is to be wound upon a core or core tube 2 intoa coil is contained in or stored upon a spool 3 which is mounted y thebracket 8, the brake 9, and the spring 10.

upon a shaft 4. The wire 1 is properly tensioned and aligned as it comesfrom the spool by the pulleys" 5, 6, and 7 and a brake' rne'chansmcomprising the pulley 6, The bracket 8 is mounted to rotate about theshaft 4 and is coupled by means of gears (not shown) to the shaft 11 towhich the brake 9 is attached. As the tension in the wire 1 tends toincrease, the force exerted by the wire 1 upon the bracket 8,transmitted through the pulley 6, causes a diminution in the pressurebetween the brake 9 and the brake disc 12 whereby the tension in thewire is decreased. On the other hand, as the tension in the wire 1 tendsto decrease, the pressure on the brake band 12' is increased whereby thetension in the wire is increased.

The wire is passed through an electrophoretic applicator 13 whereby acoating or a layer of a coating material is applied thereto. Thisapplicator is shown in detail in Fig. 3. The coating material is pumpedfrom the reservoir 31 through the pipe 32 by the pump 33 into thechamber 34 of the applicator 13. The applicator 13 essentially comprisesa cylindrical tube 35 which, with the walls 36, 37, 38, and 39, definesthe chambers 34, 40, and 41. The wire 1 is passed through the apertures42, 43, 44, 45 in the walls 36, 37, 38, and 39. The coating materialrises in the pipes 46 and 47 until the overiiow point is reached andthen flows back to the reservoir 31 via the pipe 43. A portion of thecoating material flows from chamber 34 through the apertures 43 and 44into the chambers 40 and 41 from where it flows under the influence ofgravity through the pipes 49 and 50 and the manifold 51 to the reservoir31. In coating the wire as it passes through the chamber 34, advantageis taken of the electric charge on the dispersed particles of thecoating material. For example, if the particles in the dispersion arepositively charged, that is, cationic, the wire may be grounded, forexample, by grounding the pulley 7 if this pulley is of a conductingmaterial, and the cylindrical portion 35 of the applicator 12 chargedpositively by means of a battery. Under the influence of theelectrostatic field thereby set up between the wire 1 and thecylindrical portion 35 the particles of the dispersion will be attractedtoward and adhere to the wire 1. Similarly, if the particles arenegatively charged, that is anionic, the cylindrical portion can bebiased negatively with respect to the wire 1 and the particles willlikewise adhere to the wire ll.

The electrode position type of applicator shown in Fig. 3 is furtheradvantageous in that it affords a con- Venient method of providing anelectrical connection to the coil at prescribedcpoints in the winding.The momentary destruction of the electrostatic field between the wire 1and the cylindrical portion 35 for example by disconnecting the batteryby means of a switch in the battery circuit, will result in a shortlength of wire passingv through the applicator 12 without being coated.A bare section of the wire formed in this manner can be brought out as aloop from the winding to the terminal block of the coil and secured toit. Only the insulated portions of the loop are within the body of thewinding.

The wire l is pulled from the spool 3, over the pulleys 5, 6, and 7through thev applicator 13 and wound about the core or core tube 2 intoa coil by the winding machine 14. The core tube 2 is seated on the arbor15 which is mounted for rotation upon the platform 16. The arbor shaft17 is appropriately coupled to the driving motor 18 also mounted uponthe platform 16. The platform 16 oscillates laterally, riding uponthetrack 19 and being actuated by the motor 2t), which acts through thegear box 21 and is controlled by the reversing switch 22. Thus the arbor15 moves laterally to and fro as the winding proceeds and the coil iswound in layers producing a structure such as is shown in Fig. 5.

The coating may advantageously be partially solidified prior to theactual winding of the coil in order that it may have suflicient rigidityto provide the essential mechanical separation of adjacent turns of thecoil. This partial solidilication may be accomplished in the case of athermosetting resin, among other means, by passing the wire through aheating element 23, which subjects the wire to a gaseous atmosphere at atemperature which may be as high as 800 C. depending on the chemicalnature of the resin and the speed of Wire travel. Other solidifyingprocesses, partial or complete, may include the freezing of athermoplastic resin applied from a molten iiuid bath or the coalescingof a powder by heat followed by freezing.

The final soliditication of the coatings in the coil results in abonding of the adjacent turns, producing a coil having a rigidstructure. This solidiiication is accomplished by the application ofheat to the coil as the Winding proceeds. This heating is advantageouslyproduced, among other possible means, by directing a jet of a gas, suchas nitrogen, of a temperature of the order of 120 C. toward the arbor bymeans of a nozzle 24 as the winding proceeds.

The nozzle 24 is so mounted that it can follow and at all times bedirectly over the coil arbor during the entire coil winding process.This situation is conveniently accomplished by mounting the nozzle onthe platform on which the coil arbor is mounted. The partially woundcoil acts as a heat reservoir in Which heat transferred from the hot gasjet is stored to be subsequently transferred to the more recently Woundturns of the coil. The iinal curing produces a fusion or bonding ofadjacent layers of coating resulting in a rigid mass of insulatingmaterial 25 with the turns 26 of the coil 27 distributed therein asshown in Fig. 5.

It is understood that the entire process, that is, the coating andWinding, proceeds at coil winding speeds.

After the coil is completed, the core or core tube 2 having the coil 27Wound thereon is removed from the arbor and is ready for furtherassembly, if required, prior to installation.

Another type of applicator which may conveniently be employed with theapparatus of Fig. 2 is the roller coater shown in Fig. 4 which comprisesa roller 60 having a plurality of different sized grooves 61 therein,mounted upon a shaft 62 for rotation Within a housing 63. A scraper 64,tensioned against the roller 60 by the spring 65, removes the excess ofcoating material from the roller 60 as it rotates partially immersed ina bath of coating material contained in the housing 63. The wire 1 ispassed through one of the grooves 61, as the roller 60 is rotated bysome external drive coupled to the gear 66, which is mounted, with theroller 60 on the shaft 62.

ln another embodiment of this invention utilizing aV primary insulatingcoating the wire is coated, for example, by a roller type applicatorsuch as is shown in Fig. 4 with a coating of resinous material such aspolyester' containing 30 per cent styrene to which is added 1 per centof a catalyst such as 50 per cent benzoyl peroxide, 50 per centtricresyl phosphate, which plastic is cured by passing the wire througha heating element similar to the heating element 23. The Wire, havingthe cured plastic coating thereon, is then passed through anotherapplicator which applies a coating of a casting resin such aspolyester-styrene solution containing about l per cent of a catalystsuch as 50 per cent benzoyl peroxide, 50 per cent tricresyl phosphate,over the plastic coating. The above-indicated casting resin is cured byheating the coil as it is wound upon the core tube 2 on the windingmachine 14, for example, by a jet of nitrogen of a temperature ofsubstantially 120 C. emitted by nozzie 24.

The resulting coil has a rigid structure, the essential mechanicalseparation and electrical insulation being afforded by the cured plasticcoating while the rigidity of the structure is effected by the bondingof adjacent turns of the coil by the setting of the casting resin.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. The method of producing a conducting coil having its turns embeddedin a substantially solid homogeneous matrix composedof a dispersion ofinsulating particles in a solidified resinous mass which comprisescoating a bare conductor with a Huid resinous material having solidparticles of insulation dispersed therein, solidifying the fluidresinous material of the coating to a degree sufiicient to maintain saidparticles on the conductor yet insuicient to maintain a separationbetween the wound turns of the conductor, then Winding the conductorinto a coil, the solid particles separating adjacent turns, thencoalescing the resinous material of the coatings of adjacent turns ofthe conductor, and solidifying the coating completely.

2. The method of producing a conducting coil which comprises coating abare conductor with a fluid thermosetting resin having finely dividedsolid insulating particles dispersed therein, solidifying the resin ofthe coating to a degree suliicient to maintain said particles on theconductor yet insuiiicient to maintain a separation between turns ot thewound conductor, then winding the conductor into a coil, maintaining aseparation between the adjacent turns of the Wound conductor by thesolid particles in the coating, then coalescing the resinous inaterialof the coating of adjacent turns of the conductor, and solidifying thecoalesced coatings.

3. The method of producing a conducting coil which comprises coating abare conductor with a mixture of a high butadiene-styrene co-polymervand finely divided solid particles of insulating material, bothdispersed in acrylonitrile, solidifying the coating to a degreesufficient to maintain said particles on the conductor yet insuficientto maintain a separation between Wound turns of the conductor byapplying heat thereto, then winding the conductor into a coil, the solidparticles separating adjacent turns, then coalescing the co-polymer ofthe coatings of adjacent turns, and completely solidifying the coating.

References Cited inthe flle of this patent UNITED STATES PATENTS1,595,838 Turner Aug. 10, 1926 1,606,393 Apple Nov. 9, 1926 1,826,297Apple Oct. 6, 1931 1,842,648 Bartel Ian. 26, 1932 1,924,311 Frey Aug.29, 1933 1,975,750 Saiord Oct. 2, 1934 2,293,951` Seastone et al Aug.25, 1942 2,350,822 Robinson June 6, 1944 2,394,047 Elsey et al. Feb. 5,1946 2,484,214 Ford et al. Oct. 11, 1949 2,553,362 Dannenberg May l5,1951 2,553,666 McKechnie May 22, 1951 2,561,462 Compton et al. July 4,1951 2,595,791 Hunt May 6, 1952 FOREIGN PATENTS 536,703 Great BritainMay 23, 1941

2. THE METHOD OF PRODUCING A CONDUCTING COIL WHICH COMPRISING COATING ABARE CONDUTOR WITH A FLUID THERMOSETTING RESIN HAVING FINELY DIVIDEDSOLID INSULATING PARTICLES DISPERSED THEREIN, SOLIDIFYING THE RESIN OFTHE COATING TO A DEGREE SUFFICIENT TO MAINTAIN SAID PARTICLES ON THECONDUCTOR YET INSUFFICIENT TO MAINTAIN A SEPARATION BETWEEN TURNS OF THEWOUND CONDUCTOR, THEN WINDING THE CONDUCTOR INTO A COIL MAINTAINING ASEPARATION BETWEEN THE ADJACENT TURNS OF THE WOUND CONDUCTOR BY THESOLID PARTICLES IN THE COATING, THEN COALESCING THE RESINOUS MATERIAL OFTHE COATING OF ADJACENT TURNS OF THE CONDUCTOR, AND SOLIDIFYING THECOALESCED COATINGS.